In many wireless communication systems that perform block processing, it is typically necessary that metrics for an entire block be stored in the receiver before demodulation/decoding procedures can be performed. This requires the storage of a substantially large set of metrics. Furthermore, if soft-decision combining is performed, it is necessary to represent each metric using several bits. In some digital signal processors, as much as a 32-bit word may be necessary for each metric if they cover a large dynamic range.
As an illustration of the problem existing in prior art, two example applications are shown in FIGS. 1 and 2. In the first example in FIG. 1, a block of N symbols out of a convolutional encoder is block-interleaved at the transmitter 102 in order to provide robustness against fading. At the receiver 104, the whole block of N symbols must be stored into memory before the de-interleaving process can be fully completed, at which time the decoding/demodulation procedure can begin, as indicated by the time line 106. If a soft-decision decoder is used, such as a Viterbi decoder, then each stored metric will comprise several bits. For large values of N, the amount of storage required could be prohibitively large.
In the second example, shown in FIG. 2, a frequency-hopped digital communication system employs diversity by transmitting a block of symbols twice, on each of two different hops. In the receiver, a symbol is demodulated by soft combining of the symbol's received statistics on each of the two hops. In the transmitter, a block of N symbols is sent during a time interval of Thop seconds, on two different hops. At the receiver, once the statistic from the first hop is obtained at time t=τ, the receiver must store that statistic until time t=τ+Thop, when the statistic from the second hop is received, and soft-combining can then be performed. Thus, the entire set of N statistics received on the first hop must be stored before soft combining can be done. Again, the amount of storage required can be problematic for large values of N.
Another problem is the mitigation of interference in a system employing soft combining in the receiver, which is of particular importance in the previous frequency-hopping example. This situation is illustrated in FIG. 3 for an exemplary system employing 8-FSK modulation and second-order diversity, demodulated with non-coherent square-law combining. The demodulation is performed with a matched-filter bank, one filter for each of the 8 frequencies. A strong interference process corrupts the matched-filter metrics from the first hop 302. Those from the second hop 304 contain a relatively strong signal component, although the power in that signal is significantly lower than the interference power on the first hop 302. Thus, when the metrics are square-law combined, the interference process overwhelms the resulting set of metrics 306, and there is essentially no signal component.